1. Field of the Invention
This invention relates to methods of adjusting the inductance of inductive devices and, more particularly, to those of the sectioned core type wherein the core substantially enclose the coil means and forms a central air gap within the latter, and to methods of adjusting the inductance of such devices.
2. Description of the Prior Art
In many demanding electronic circuit applications, inductors or transformers often are required which utilize magnetic cores, such as of the ferrite type. As is well known, ferrite cores may be manufactured to exhibit a wide range of magnetic properties for a particular circuit application, such properties ranging from a very high permeability (thereby being readily magnetized) to a very high coercive force (thereby having the capability of retaining magnetism). Ferrites also exhibit very high electrical resistivity, and it is this latter property that makes them particularly useful as cores in many inductive circuit applications when compared to either magnetic iron or alloys. Ferrites are manufactured starting with a desired proportionate mixture of oxides of iron and zinc, together with manganese carbonate, which mixture is thereafter compacted or pressed, and then fired to form the desired rigid geometric core configuration.
While the art of manufacturing ferrites is relatively old, it is nevertheless a rather difficult and complex process, involving a number of critical variables that must be controlled in a very exacting manner in order to attain even reasonable uniformity of product. These variables relate not only to the preciseness of the percentage concentrations of the basic constituents, but to other factors such as the pressing and firing conditions employed during their manufacture, as well as the presence of even small amounts of impurities, and any possible exposure to foreign contamination.
Compounding the problem of achieving uniform electrical and magnetic characteristics in ferrite cores heretofore has been the dimensional variations encountered in the fired cores. More specifically, ferrite cores must initially be pressed into the desired shape with oversized dimensions, typically ranging from ten to twenty per cent, so as to compensate for the subsequent shrinkage thereof during firing. As such, it becomes readily apparent that whenever very close tolerances are required with respect to critical core dimensions, such as an air gap formed in sectioned cores, abrasive machine grinding or lapping operations have often been required heretofore.
Such auxiliary operations are both time consuming and expensive, and have been particularly required in connection with the manufacture of cup-shaped ferrite cores. Each section of such cores is generally formed with a circular end wall, either a continuous or segmented outer annular wall and an axially disposed and inwardly extending tubular leg portion that defines an inner wall. The annular space defined between the inner and outer walls of each core section accommodates a portion of the coil (or coils) surrounding the leg portion thereof, with the latter terminating relative to the end of the mutually disposed leg portion of the mating core section so as to define a pre-established air gap therebetween.
As thus constructed, it is readily appreciated that the relatively thin outer walls of the mating cup-shaped core sections provide a substantially enclosing housing for the coil(s), and an effective, low reluctance return path for magnetic flux. Unfortunately, however, such cup-shaped cores result in the pre-established central air gap being substantially inaccessible for machine grinding after the assembly of the inductive device. It is for this reason that the peripheral annular wall edge and/or the terminating end of the leg portion of at least one of the core sections has had to be machine ground heretofore before their assembly so as to establish an initial air gap, the width of which is within predetermined limits. Only in this way could the initial value of inductance exhibited by the device be expected to fall within a predetermined acceptable range.
However, even with this added expense in device manufacture, the typical tolerance variations encountered in both the coil(s) and core sections have still normally required a ferrite tuning slug (and associated split sleeve) to fine tune the assembled device. Such a slug is employed to adjustably control the reluctance across the air gap and, thereby, the inductance, when the device is used in demanding circuit applications.
Of course, should the initial machine grinding operation not have resulted in the air gap spacing falling within a range that would allow the tuning slug to adjust the inductance of the assembled and energized device within acceptable limits heretofore, either the completely assembled unit has had to be discarded, or the device disassembled, with either one or both of the core sections being subjected to further machine grinding, if practicable, or selectively replaced by new ones. As the core sections are often cemented together during assembly, and as a plurality of coils are generally employed, which typically necessitates that the lead-out wires therefrom be wire-wrapped or solder-connected to associated terminal posts, any disassembly operation has been time-consuming and expensive.
In order to dynamically adjust the inductance of inductive devices utilizing a substantially different core heretofore, namely, a solid ferrite core of the toroidal type, R. L. Weber U.S. Pat. No. 3,548,492 discloses the use of an air stream of abrasive particles to either partially or completely form an initial air gap through an exposed area of such a core in order to change the reluctance of the core and, thereby, the inductance of the device. In assembled inductive devices utilizing cup-shaped cores, however, the terminating ends of the core leg portions that define the central air gap are not readily accessible for machining by a stream of abrasive particles. As such, an abrasive stream could not be effectively utilized either to form such an internal air gap initially, or to modify the air gap after the assembly of the device for the purpose of adjusting the inductance thereof.
Moreover, any attempt to increase the diameter of the bore in one or both tubular leg portions (adapted heretofore to receive a tuning slug) so as to allow an air stream of abrasive particles to be directed therein at a small angle relative to the axis thereof, in an attempt to remove core leg material defining the air gap, would not prove to be very practicable. Such a process would create a substantial amount of detrimental debris in the form of both core material and abrasive particles (as none of such material would be vaporized), and would be relatively slow. In addition to such problems, in order for an abrasive stream to be very effective in removing core material, the angle of incidence thereof should be as low as possible relative to the core surface being machined. In this regard, there is no practical way to deflect an abrasive stream at the requisite angle after entering the bore of one of the core leg portions, at least not with the abrasive stream remaining concentrated and effective for machining purposes.
As for using a stream of abrasive particles to form a partial or complete air gap in an outer exposed area of an enclosing core, the relatively thin outer walls of such core sections do not permit the type of air gap disclosed and appreciated in the Weber patent. Moreover, any attempt to utilize abrasive particles to form any type of slot in such a wall would be very messy, lack precision control over slot definition, render the thin wall-particle-blasted core section particularly susceptible to fracture, and be time consuming.
In Hartwig Lohse U.S. Pat. No. 3,874,075, a high energy beam source is disclosed for cutting a plurality of helical grooves in a metallic coating applied to a solid rectangularly-shaped core so as to produce an inductive device. This reference also suggests that a groove or notch, which may ultimately extend through the core to form a complete air gap, may be employed to adjust the inductance of the device, and that such a notch or air gap may be formed in a region removed from the externally exposed winding by the use of either a high energy beam, sand blasting, grinding, or other suitable means.
As the solid-centered construction of the rectangularly-shaped core disclosed and of concern in the Lohse reference is essentially the same as the toroids disclosed by Weber, it is not surprising that Lohse appreciated that sand blasting, as well as a laser beam, could be utilized to form either a partial or complete air gap in such a core without either cracking the core, or endangering the external windings that are physically spaced from the air gap.
None of such prior art, however, suggests or provides an understanding of how the inductance of a completely assembled inductive device, having a pre-established central air gap and coil assembly, both of which are substantially enclosed by relatively thin, outer core walls, could be precisely adjusted dynamically not only without the need of a tuning slug, but without fracturing the core, and/or adversely affecting the coil assembly housed therewithin.